This invention relates generally to a heat exchanger, and more particularly to a heat exchanger for use with a kiln or the like for raising the temperature of combustion air by utilizing the heat of the exhaust flue of the kiln.
While a heat exchanger according to this invention is useful in a variety of applications, the description will be facilitated by addressing the problem of providing a heat exchanger for application to high temperature pottery kilns of the type generally used by studio potters, schools, laboratories, hobby ceramists, clay product factories and pottery factories.
In a typical kiln, the heat from spent exhaust gases is essentially wasted as it simply exits the flue and goes up a chimney or the like. Thus, increased fuel efficiency is possible if this waste heat can be utilized to pre-heat combustion air. In this way, a larger percentage of the heat energy of combustion is utilized in raising the temperature of the product being fired and a correspondingly smaller percentage of the energy is required to raise the temperature of the air entering the combustion chamber.
A number of problems arise in attempting to utilize the heat of exhaust gases in this fashion, and devices known to the prior art have generally failed to solve these problems and to maximize fuel efficiency. For example, for maximum efficiency it is desirable to heat the air for combustion to as close to the temperature of the firing chamber as possible. However, in high temperature pottery kilns, this temperature is typically in excess of 2000.degree. F., but the materials used in many prior art devices are not suitable for withstanding such high temperatures. Additionally, to attain such a high air temperature, it is necessary for the air to be in thermal contact with the hot exhaust gases of the flue, for a substantial amount of time, over a substantial area, or both. In contrast, many prior art devices provide only relatively short pipes or the like in contact with flue gases, whereby it is impossible to attain the high temperatures required for maximum efficiency in pottery kilns of the type mentioned. Also, it is necessary to confine combustion of the fuel and air entirely to the combustion chamber of the kiln, and prevent fuel from mixing with the heated air prior to the air reaching the combustion chamber, to prevent ignition or combustion thereof in the air carrying tubes or the like prior to reaching the combustion chamber. Further, to obtain the required heating of the air, it is necessary to provide thermal contact between the air carrying conduits or the like and the flue not only in the relatively cooler portions of the flue, but also in the hotter portions thereof, extending as close as possible to the hottest end thereof, which communicates with the firing chamber. Again, many prior art devices utilize materials not suitable for this type of an arrangement.